Yes, thermometers were invented around that time. But to record GLOBAL TEMPERATURES, you need two thing: 1) an accurate thermometer. Yes, they existed prior to 1980. But you also need 2) global measurements. Prior to satellite measurements, there were very large parts of this globe that didn't have any measurements at all. There were accurate thermometers, they just weren't located all over the place.
And, surprise, the satellites, when they are correctly calibrated, confirm the thermometer data.
In a qualitative sense, not getting the small area over the poles is MUCH different than not getting the vast majority of the planet at all.
ignoring the little problem that the poles may be warming faster than the rest of the planet.